The OTFS overlay for OFDM as presented in [1]}, [2]}, [3]}, and the continuous Zak transform approach in [4]} are based on analog representations of the modulation and demodulation. The analog representations offer a framework to study OTFS and its properties. However, they are ill-suited for practical implementation. For instance, the hardware effort for analog OFDM is prohibitive, as each subcarrier would require a local oscillator [5]}. Similarly, the OTFS modulation presented in [4]} maps each symbol in the DD domain on an orthogonal basis function. The number of symbols defined on a grid in the DD domain is typically in the order of \(10^3-10^4\)  (see for example [3]}) or [2]}). Thus, a direct implementation of OTFS based on the continuous Zak transform exceeds the hardware effort of the analog implementation of OFDM and makes it even less suitable for direct implementations.
